feat: add keyboard navigation

[ryanshanz]

Following up on issue #404. I've added moving to the next comment ('j' key), previous comment ('k'), next thread ('t'), and previous thread ('p'). Some of the other comment navigations were a little unclear to me, so I stuck with these ones. Feel free to let me know if any of them look a little off. I also added in link navigation ('Enter' or 'shift' + 'Enter' for new tab) because that seemed super quick.